Semiconductor optical amplifiers (SOA) are conventionally used as optical switches, whereby the output of light from the SOA is switched OFF and ON by switching the drive current supplied to the SOA OFF and ON (see, for example, Japanese Laid-Open Patent Publication No. 2001-154160).
Also conventionally used are speed up circuits that are provided upstream of a load having reactance and that speed up the rise of current flowing through the load by shaping the waveform of the voltage applied to the load and enhancing the edge (the rising portion). A load having reactance is a load in which the rise of the flowing current is delayed with respect to the rise of the applied voltage.
An example of a speed up circuit is a speed up condenser connected in parallel with a resistor connected in series with the load (see, for example, Japanese Laid-Open Patent Publication No. 2005-229402). Another example of a speed up circuit is a speed up inductor connected in series with a resistor having a branched connection to the load.
However, the conventional speed up circuits described above cannot shape the rise of the waveform of the supplied current accurately due to variation in the self-resonant frequency of the condenser and/or the inductor. Specifically, the condenser has parasitic inductance due to manufacturing variation, and variation in the parasitic inductance causes variation in the period during which the applied voltage is enhanced. The inductor also has parasitic capacitance due to manufacturing variation, causing variation in the period during which the applied voltage is enhanced.
For example, when the load is an SOA used as an optical switch, the variation in the period during which the applied voltage is enhanced causes variation in the rise time, thereby preventing highly-accurate switching. Variation in the self-resonant frequency of the condenser or the inductor particularly affects the SOA since the supplied current is turned ON and OFF on the order of nanoseconds.